This invention relates to atomic emission spectroscopy and more particularly to a background compensation apparatus and technique for an atomic emission spectrometer for multi-element measurement of elements in a sample.
Atomic absorption spectroscopy (hereinafter "AAS") is a known procedure for measuring the concentration of a certain element in a sample. AAS utilizes the fact that atoms absorb light at certain wavelengths characteristic of the particular element. Atoms emit light in the form of a line spectrum when they are excited and the line spectrum is characteristic of the respective element. Correspondingly, the atoms absorb light only at the wavelengths of this line spectrum.
In AAS, an atomizing device in the form of a flame burner or a graphite furnace for electrothermal atomization is utilized to generate an atomic vapor of the sample in which the atoms of the sample are present in their atomic state. A measuring light beam is normally generated by a hollow cathode lamp and consists of light with the line spectrum of the looked-for element. This measuring light beam is passed through the atomic vapor and is subjected to absorption indicative of the amount of the looked-for element in the sample. The other components of the sample, at least theoretically, do not influence the measuring light beam because their absorption lines do not coincide with the line spectrum of the measuring light beam. The measuring light beam impinges on a photoelectric detector and the concentration of the looked-for element is determined from the detector signal after suitable processing and calibration.
In addition to the specific absorption caused by the atoms of the looked-for element (i.e., atomic absorption), there normally occurs a non-specific absorption referred to as background absorption which is caused by solid particles or molecules in the path of the measuring light beam. This background absorption can have the magnitude of the atomic absorption or can even be larger. Therefore, background absorption has to be determined and taken into consideration with highly sensitive measurements. In AAS, background absorption correction can be achieved by alternating between the line emitting light source and a light source emitting a continuum or by using the Zeeman effect in which either the emitted spectral lines of the light source or the absorption lines of the sample are shifted for the background measurement. These methods are quite expensive and require an additional light source or a strong electromagnet which is arranged to be energized or de-energized.
A disadvantage of AAS is that the elements can only be determined one by one, i.e., one after the other.
Therefore, another known analytical procedure is to measure the emission of a sample rather than the absorption, i.e., atomic emission spectroscopy, which allows multi-element measurement.
In atomic emission spectroscopy, plasma burners are often used as the atomization and excitation device. In plasma burners, an emerging inert gas is inductively transformed to a plasma of high temperature and the sample is led into this plasma. In another prior art atomization and excitation device, a sample is electrothermally dried and ashed in a graphite furnace similar to the graphite tubes used in AAS. The graphite furnace is then evacuated and an inert gas is introduced. Subsequently, an electrothermal atomization of the sample is effected. A gas discharge is caused in the mixture of inert gas and sample vapor by an anode such that the graphite tube operates as a hollow cathode lamp. The graphite tube serves as a hollow cathode.
A spectrum of the emitted light is generated by means of a polychromator. It is known to scan such a spectrum by means of a series detector or "detector array" consisting of a plurality of photodetectors. The entire spectrum is detected which results in a great amount of data and the signal processing is correspondingly complex.
A polychromator is known in which a dispersion is effected in high order in a first direction by an echelle grating. The different orders overlap and a dispersion is effected in a second direction perpendicular to the first direction by a dispersion prism whereby the different orders are separated. This results in a two-dimensional spectrum with very high resolution in a focal plane.
In the prior art polychromator, a mask with apertures at the location of the spectral lines of the spectrum which are characteristic of a certain element is arranged in the focal plane. These apertures are arranged to accommodate light pipes, each of which is guided to an associated photomultiplier. The number of available photomultipliers and thus the number of elements which can be analyzed simultaneously is therefore necessarily limited due to cost considerations. One mirror of the polychromator is movable through small angles to compensate for the background emission which occurs in a similar way as the background absorption described above. The generated spectrum is periodically shifted relative to the light pipes such that the light pipes detect the light outside the spectral lines.
It is an object of the present invention to provide a new and improved atomic emission spectrometer with background emission compensation.
Another object of the invention is to provide such an atomic emission spectrometer for multi-element measurement which attains simultaneous measurement of a relatively large number of elements with simultaneous measurement and correction of background emission.
Another object of the invention is to provide a background emission correction device and technique for an atomic emission spectrometer which is economical.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, it has been found that the foregoing and related advantages are attained in an atomic emission spectrometer for multi-element measurement which includes an apparatus to atomize a sample and excite the atoms to emit characteristic spectral lines, a dispersion assembly to generate a spectrum of characteristic spectral lines in a focal plane, a first photodetector assembly for simultaneously sensing the intensity of spectral lines of a plurality of elements, a second photodetector assembly for sensing background emission outside the spectral lines sensed by the first photodetector assembly, and processing circuit means for determining the concentrations of the plurality of elements with correction for background emission.
The second photodetector assembly has a plurality of semiconductor photodetectors disposed for sensing background emission adjacent the spectral lines for generating a correction signal which corresponds to the background emission at the wavelengths of the spectral lines measured. The first photodetector assembly may comprise a plurality of semiconductor photodetectors positioned for simultaneously sensing a plurality of spectral lines for each element of the sample to be tested and an evaluation circuit selects the semiconductor photodetectors sensing the spectral line for each element which has an intensity optimally within the sensing range of the respective semiconductor photodetector. The measured intensity is corrected with the background emission correction signal with the concentrations of the elements being determined from the corrected intensities.